When a fluorescent lamp approaches the end of its operating life, the electrode emission capability of at least one of its cathodes decreases, accompanied by a corresponding increase in the voltage drop across that cathode. The increased voltage drop caused increased power dissipation in that cathode and a potentially significant increase in the temperature of the lamp in the area of that cathode. The increase in temperature is especially pronounced in small diameter lamps (such as T5 lamps) because those lamps have a smaller surface area and a larger operating current in comparison with larger diameter lamps (such as T8 lamps).
The localized high temperatures that often occur in small diameter lamps during end-of-life conditions present a potentially serious safety hazard. Accordingly, ballasts for powering small diameter lamps require some form of protection circuitry for detecting and responding to end-of-lamp-life conditions. Although the prior art is replete with approaches for protecting ballasts under various lamp fault conditions, a continued need exists for economical circuits for protecting against hazards that accompany end-of-lamp-life conditions.